Dichloroacetaldehyde (hereinafter, is referred to as "DCA") is useful as a synthesis raw material for medicines or agricultural chemicals and a method of producing DCA from para-acetaldehyde, acetaldehyde, or 1,2-dichloroethylene by chlorination is known. However, since DCA is a very unstable compound and very easily polymerized, it is difficult to store DCA for a long period of time as is. Accordingly, to insure stability, DCA is stored as an aqueous solution thereof or an acetal thereof but in such a case, it is difficult to obtain high pure DCA as a synthesis raw material from the aqueous solution or the acetal.
As a method capable of stably storing DCA in stable form and reproducing high pure DCA, an attempt to form a trimer of DCA is disclosed by David D. Centola et al, in U.S. Pat. No. 3,322,792. According to the method, DCA containing less than 1% by weight monochloroacetaldehyde as an impurity is heated to a temperature of from about 10.degree. C. to about 70.degree. C. in the presence of a metal chloride such as antimony chloride, iron chloride, aluminum chloride, tin chloride, etc., and chlorine.
However, since it is difficult to separate monochloroacetaldehyde from DCA, for preparing a mixture of chlorinated acetaldehydes, it is necessary to subject a chlorinated liquid composed of about 85% DCA, about 13% monochloroacetaldehyde, and about 2% chloral obtained by para-acetaldehyde or acetaldehyde under an ordinary condition, for obtaining DCA, to a chlorination reaction in a presence of para-toluenesulfonic acid and antimony trichloride as the catalyst as described in U.S. Pat. No. 3,150,189. However, in this method, since a metal chloride is used to catalyze the trimerization, the separation or removal of the metal chloride is troublesome and further when antimony trichloride having a relatively high trimerization yield in these metal chlorides is used, there is a problem in the recovery of non-toxic materials.
Also, as other method of forming a polymer of DCA, it has been noted that a very slight amount of a DCA polymer is formed by reacting DCA or DCA diethylacetal and sulfuric acid by Oscar Jacobsen, Chemische Berichte, 8, 87-88(1875) but, as a matter of course, the method can not give a yield in an amount sufficient for an industrial production process.
This invention is a novel process of producing a DCA trimer, which has been made after considering the aforesaid processes.
An object of this invention is to provide a process of producing a DCA trimer wherein the high pure DCA trimer is obtained in a high yield.
Also, another object of this invention is to provide a process of producing a DCA trimer without the need for using DCA having a low content of impurity (monochloroacetaldehyde) obtained by a specific chlorination reaction as a raw material.
Furthermore, a still further object of this invention is to provide a process of producing a DCA trimer which can be easily purified without using a metal chloride, etc., as a catalyst.
A further object of this invention is to provide an industrially useful process of producing a DCA trimer, wherein concentrated sulfuric acid used for the production of the DCA trimer can be recovered by a simple method for repeated use.